The present invention relates to a reproducing system for video tape recorders and, more particularly, to a reproducing system for video tape recorders where a plurality of reproduced video signals sequentially reproduced by a plurality of video heads are alternately switched to produce a continuous reproduced video signal.
Generally, a home-use video tape recorder of the two rotary head type is provided with a disc having two video heads disposed on the periphery thereof with an angular separation of 180.degree.. The disc is rotatably disposed within a slit of a cylinder with a slightly larger diameter than that of the disc so that the head chips of the two video heads project outwardly from the outer periphery of the cylinder. A magnetic tape is wound around the cylinder, lapping the outer wall of the cylinder over a range of 180.degree. or more in a .OMEGA.-shaped fashion. The disc is rotated at a speed of 30 rotations per second while the magnetic tape is transported at a constant speed in an opposite direction to the rotating direction of the disc, slanting with respect to the slit of the cylinder. In this way, the two video heads alternately record onefield video signals of each picture on the magnetic tape in slant recording tracks during each 1/60 second, so that the video signal recorded on each slant recording track has its both end portions overlaped with end portions of the signals recorded on the adjacent slant recording tracks. Use is made of a servo system for making the phase of the vertical synchronizing signal concurrent with the rotation phase of the video heads in order that the vertical synchronizing signal included in the video signal is recorded on the overlapping portions of the tracks.
In reproducing the video signals, the two video heads alternately and accurately trace the corresponding tracks on the magnetic tape to pick up the video signals recorded thereon. The signals picked up are amplified by their associated preamplifiers to form reproduced channel signals which are fed to corresponding channel switch circuits where the channel signals are subjected to a switching control at the overlapping portions at every 1/60 second. A series of continually reproduced video signals each for one field produced from the switch circuits are then coupled by partial superposition to be a continuous video signal. The video signal includes an FM signal and a color signal. The FM signal is a signal of 3.4 to 4.4 MHz frequency-modulated with a brightness signal of 0 to 4 MHz while the color signal is a signal of 629 KHz which is a conversion from a chromaticity representing signal of 3.58 MHz.
As described above, the home-use video tape recorder uses two video heads to alternately reproduce the video signals. However, since there exist inherently some variations in the level of signals reproduced by a video head when two heads are used in combination, the level variation is more likely to occur, which may cause undesirable phenomena such as, for example, flicker.
For this reason, in the conventional system, the outputs from the channel switch circuits are not directly summed but summed by using an adder including a variable resistor. Specifically, both fixed ends of the variable resistor are connected as input terminals to the output terminals of the two channel switch circuits and the movable terminal of the variable resistor is used as an output terminal. The movable terminal is manually operated to change the resistance ratio in order that the levels of output signals, particularly FM signals, from the video heads are adjusted to be made equal to each other. Through this adjustment, the channel level difference between the two video heads are adjusted and then the reproduced FM signal is applied to an amplifier at the next stage where it is properly amplified to have a proper level. The FM signal with the proper level is supplied to a level adjustor to adjust the level of the FM signal to be inputted to a dropout compensating circuit at the next stage. The dropout compensating circuit is used for compensating for a dropout possibly occuring in the video signal. The level for the dropout is generally selected to be the one approximate to -20 dB of an ordinary average reproduced level. Therefore, if the reproduced FM output changes, the dropout level also changes correspondingly. When this change of the level of the reproduced FM signal is large, even a slight level reduction operates the dropout compensating circuit so that the switching noise is rather conspicuous. On the other hand, when it is small, the dropout compensation is almost ineffective. It is for this reason that the reproduced FM output level is kept constant by the level adjustor. Meanwhile, the color signal is taken out from a succeeding stage to the adder, i.e. to the channel balance resistor, through a low-pass filter. The color signal is controlled to have always a constant level by an automatic color level control circuit (ACC circuit) connected at a succeeding stage. Accordingly, the color signal suffers from no problem in particular.
As described above, conventionally, the channel balance and the level adjustments are manually performed, taking a relatively long time, which is a disadvantage. Further, these two adjustments are made to be settled at an optimum point of the reproduction system used. Therefore, when a magnetic tape recorded by another system is used for reproduction by this reproducing system, which has been previously adjusted (such a reproduction being called an interchanged reproduction), variations of channel levels and the reproduced FM output levels may take place or the dropout compensation may be unstable, resulting in deterioration of picture quality.
Further, since a variable resistor is used for the two adjustments, when the reproducing system is fabricated as an IC, the resistor cannot be assembled into the IC, that is, the resistor must be used separately from the IC. Therefore, the IC needs additional pins to connect the resistor.